It is well known to locate electric power plants in close proximity to a facility requiring heat whereby the waste heat from the generation of electricity may be utilized with minimum transportation cost and minimum loss in the heat distribution system. Such cogeneration plants are commonly found in large facilities such as large buildings or complexes of buildings. They generate electricity for their own use and, at times when an excess is produced, the excess power is sold to the power utility for distribution to its other customers.
Heretofore such cogeneration plants have not been successfully applied in the single family home or small apartment building. The potential advantages resulting from generating electricity from even a small fraction of the fuel presently consumed for heating and supplying domestic hot water in small buildings are substantial. That fuel which is converted to electricity is converted at about one hundred percent efficiency in a well designed cogeneration plant whereas modern hydrocarbon burning central power generating stations may waste sixty five percent of the fuel burned. The reason there is no waste is because the cogeneration system of the invention operates only when heat is needed so none of the energy in the fuel is wasted. Central power plants and long distance distribution equipment also require very substantial capital investment.
Heretofore no combined power and heat generating plant has been successfully applied to heating such as single family residences. Many attempts have been made to adapt the designs that work well in larger sizes to the home but formidable problems have prevented commercial success. These attempts typically involve an internal combustion engine driving an electric generator with means to utilize the heat produced for space heating. One problem is that internal combustion engines generate noise levels that are difficult to insulate to the degree required at acceptable cost. Another problem is that much of the heat generated by an internal combustion engine is not easily applied to domestic heating. This is because heat from the engine coolant, heat from the engine exhaust, and heat from the exterior surface of the engine must be contained and transferred to where the heat is needed. This requires complex heat exchange apparatus which adds to the cost. Containing heat from the exterior surface of the engine is typically accomplished by insulating the engine which results in high temperatures which increases cost and reduces reliability. In domestic heating plants where heat is distributed as hot air the air to be heated cannot be in direct contact with the engine because this provides a noise conduit between the engine and the residential areas. U.S. Pat. No. 4,736,111, issued Apr. 5, 1988 to Linden teaches the making of a unit that minimizes heat radiation of the engine but the cost of doing so is significant and acoustic isolation and tight sealing are required. The prior art is more fully discussed in the aforementioned patent issued to Linden.
It is also known to use a hydrocarbon burning turbine as a prime mover. This solves a number of the aforementioned problems but known turbines are expensive. One contributor to that cost is the need to inject the correct amount of fuel into the combustor. Also, a low cost and highly efficient heat exchanger for removing heat from turbine exhaust requires a large pressure drop in the heat exchanger which requires a high pressure ratio in the compressor stage which is not easily achieved in small turbines.
Heretofore it has not been recognized that a highly efficient prime mover is not required for cogeneration plants used in small buildings. Further, it has not been heretofore recognized that low efficiency prime movers typically reduce or eliminate the aforementioned problems. Further, it has not been heretofore recognized that application of turbines to small cogeneration systems can be practicable when high efficiency is not important. Further, in the case of a turbine, if the compression is accomplished by a positive displacement pump and only the expansion is done in a turbine the problems of obtaining the correct fuel to air ratio are eliminated. Further, it has not been heretofore recognized that a prime mover comprising a positive displacement compressor in combination with a turbine expander has low noise when its exhaust is directed to a heat exchanger having a high pressure drop. Further, it has not been heretofore recognized that a prime mover comprising a positive displacement compressor and a positive displacement fuel pump inherently mixes the fuel and air in the proper proportions. Further, it has not been heretofore recognized that prime movers of low thermal efficiency operate at relatively low temperatures and have low internal stresses and are, therefore, low in cost and particularly suitable for domestic use. Finally, it has not been heretofore recognized that the turbines developed for the automotive industry for use in turbochargers provide a low cost and readily available means for converting pressurized combustion products to rotary motion in a domestic cogeneration system.